My usual frosting, the method I learned from my mother, is the standard American buttercream made with butter, icing sugar, and milk or cream (or, for that matter, orange juice or some other suitable flavoured liquid). Recently, though, I've been doing a fair bit of exploratory baking, trying out new and interesting recipes and techniques, and I decided it was time to try a real buttercream frosting.

Avjewe's writeup tells you how to make buttercream icing with egg yolks, but you can also use egg whites. I prefer this, because having used half a dozen yolks in the cake layers I'm usually looking for a good use for the orphaned whites.

Here's a recipe:

4 egg whites
1 c. (190g) white sugar
¼ tsp. cream of tartar
1 lb (450g) very soft unsalted butter
Flavourings and/or colouring as desired

A double boiler, or a metal bowl set over a saucepan of simmering water
A hand whisk (a fork probably also works)
A thermometer
A mixer (either stand or handheld), with both whisks and beaters
A large bowl (if you're not using a stand mixer)

Whisk the egg whites, sugar, and cream of tartar together in the top of the double boiler. Heat, whisking occasionally, until the mixture reaches 160°F (71°C). The sugar stops the egg from cooking, and heating pasteurizes it. Heating them both together also prevents the spectacular foam overflow that can result from pouring sugar syrup into beaten egg whites (as I discovered a couple Christmases ago while making nougat - but that's another story).

Once the egg white/sugar mixture has been heated, take it off the flame and pour it into the large bowl (unless, of course, it's already in said bowl). Beat with a whisk (preferably attached to an electric mixer) until it forms stiff peaks. If you're truly hard core, use a hand whisk. I don't have the arm muscles for it.

Once the whites are whipped, and the mixture is no longer hot to the touch, switch to ordinary beaters, and start adding the butter. The meringue mixture will fall a bit as the fat gets mixed in, and it may curdle if the butter is too cold. If that happens, keep beating; it will warm up and the icing will smooth out nicely.

At this point, proceed to add any flavours you like and beat them through: a couple teaspoons of vanilla, or perhaps coffee in the form of a tablespoon or two of instant coffee or espresso powder dissolved in the bare minimum of boiling water. Nut flavourings like almond or hazelnut work well too. I'm told fruit purees are good, although I haven't tried (yet). Up to 8 ounces (225g) of melted good chocolate (preferably as dark as possible) is, of course, an excellent option.

Final notes: You'll want to spread this while it's relatively warm; once cooled, it goes quite solid. If you want to keep it chilled to use later, it will need to come back up to room temperature. I'm told this stuff can even be frozen, although I haven't had a reason to try (why freeze frosting for later, when I could be eating cake now?)

Original source,
rewritten by me